The present invention relates generally to devices for fixation of parts of a fractured bone and more specifically, to bone plates and systems for stabilization and/or compression of parts of a fractured bone.
Bone plates may generally be utilized to carry out two different types of osteosynthesis, namely xe2x80x9crigid osteosynthesisxe2x80x9d and xe2x80x9cflexible osteosynthesis.xe2x80x9d Rigid osteosynthesis is used for medical care of joint fractures, simple shaft fractures (where nailing is impossible) as well as for osteotomies. Aside from the possibility of anatomical repositioning, the bone itself supports and stabilizes the osteosynthesis, which allows for the possibility of putting stress on the extremity earlier and without pain. Additional advantages of the medical care of stable fractures can be observed when the blood circulation in the bone is greatly diminished due to trauma. For treating xe2x80x9cnonunionsxe2x80x9d or in the case of existing infection, the fracture must be kept stable in order to make bone healing possible and so as not to irritate the infection further by instability of the fracture gap.
Flexible osteosynthesis, also known as xe2x80x9cbiological osteosynthesis,xe2x80x9d may be desirable in the medical treatment of comminuted fractures in the shaft region of tubular bones. In the case of these fractures, it is an objective to maintain the proper length of the bone and to fix the bone ends (joints) in their proper anatomic positions with respect to one another. With flexible osteosynthesis, the fracture zone is not directly affixed or manipulated, and consequently, the blood circulation in this area is not inhibited. Bone plates designed for flexible osteosynthesis thus operate similarly to a locking, intramedullary nail, which is anchored only in the metaphyses.
Since fractures cannot always be treated with one type of osteosynthesis, surgeons must frequently compromise because a bone plate, which allows him to combine the two types of osteosynthesis discussed above, is not available. Such a combination would be beneficial, for example, when a joint fracture can be compressed with traction screws through the bone plate and the whole of the joint may be connected to the diaphysis over an internal fixative with angularly stable screws. Another illustrative application concerns porotic bones, where a bone plate with axially and angularly stable screws can be anchored in the metaphysial fragment, with a stable plate-affixation being undertaken in the diaphyseal range with the assistance of a plate traction screw through the fracture. A primary fracture stabilization can be achieved by this type of procedure.
This situation has led to the development and marketing of bone implants for both types of osteosynthesis. The two types of implants, however, are designed specifically for their respective method. Thus, the disadvantages of these two systems lie in the difficulty in combining them.
Thus, a need exists for improved bone plates that provide for both rigid and flexible osteosynthesis.
The present invention is directed to a bone plate that is adapted to be used for both rigid and flexible osteosynthesis, without compromising the ability of the plate to be used for either type of osteosynthesis. Accordingly, the bone plate of the present invention may be used as a compression plate or as an internal fixative.
This objective is accomplished with a bone plate having at least one xe2x80x9ccombination hole.xe2x80x9d The combination hole may be used with a screw having a substantially spherical head to provide for compression of the fracture, or may be used with a screw having a threaded head to fix the position of the screw with respect to the bone plate and serve as an internal fixative.
The combination hole includes a first portion and a second portion that at least partially overlap one another. The first portion may be substantially circular, and the second portion may be elongated. Within the scope of the invention, the second portion (elongated portion) may have a diametrical dimension that is greater in one direction than in another. For example, the diameter of the elongated portion may be greater in the direction of the longitudinal axis of the plate than in the direction substantially perpendicular to the longitudinal axis. Thus, the elongated portion may be oval, elliptical, rectangular or any other elongated shape known to one of ordinary skill in the art, including combinations of these shapes. The diameter (D) of the first portion (circular portion) may be smaller than the minor (or shortened) axis (B) of the second portion (elongated portion). Typically, diameter (D) is between about 5% and about 25% smaller than the minor axis (B).
According to another aspect of the invention, the circular portion of the hole may be configured and dimensioned to engage the head of a bone screw. More specifically, the circular portion may be provided with an internal thread or a peripheral lamella or lip that may engage a corresponding structure formed on the screw-head. In the case where an internal thread is provided, the thread may be disposed in a single plane, or in several planes. The plane(s) may be parallel to the upper and/or bone contacting surfaces of the bone plate. According to one embodiment, the internal thread may extend over the whole height of the bone plate from the bone contacting surface to the upper surface. This configuration provides increased stability of the bone plate/screw-head interface.
With the threaded screw-head engaged in the threads of the first portion, the bone plate may be used as an internal fixative. Use in this configuration, however, creates high stresses at the interface of the bone plate and screw-head because the plate is not forced against the bone, and therefore, the bone fracture is fixed primarily by friction between the plate and the bone. This increase in stress is taken into account by the threaded portion of the hole extending over a range of at least about 180xc2x0 with respect to a central axis of the hole, and thereby enclosing the screw-head in at least this angular range. This feature of the bone plate is especially advantageous where thin bone plates are involved. Preferably, the threaded portion is disposed on one of the two longitudinal ends of the hole. This positioning allows for the threaded portion to extend over a larger angular range. For example, the threaded portion may extend over a range of between about 190xc2x0 and about 280xc2x0, and preferably over a range of between about 200xc2x0 to 250xc2x0, thus maximizing the strength of the bone screw to bone plate interface.
According to another embodiment of the invention, the internal thread may be tapered (i.e., formed on the inner surface of a hole that tapers with respect to its central axis). Preferably, the internal thread tapers radially inward toward the bone contacting surface of the bone plate. A bone screw to be rigidly fixed to the bone plate may include a screw-head having a tapered external thread (i.e., formed on an outer surface of the screw-head that tapers with respect to the central axis of the screw-head) that is tapered to match the shape of the tapered internal thread. The bone screw may be rigidly fixed to the bone plate by engagement between the matching threads. This method of attachment is especially advantageous when self-drilling screws are to be used since, due to the tapered shape of the matching threads, the screw may be inserted into the bone independently of the plate. More specifically, the screw-head becomes rigidly clamped to the plate only as the threaded screw-head penetrates the threaded portion of the hole. Despite any initial misalignment between the threads on the screw-head (the position of which are initially dictated by the orientation of the bone screw in the bone) and the threads on the bone plate, the tapered shape of the mating threads ensures that the threads on the screw-head will ultimately align with the threaded portion of the hole. When the tapered thread of the screw-head is tightened into the internal thread of the hole, the screw-head creates radial forces in the plate hole. Thus, the bone plate must be dimensioned and configured to withstand these high radial forces, e.g., to withstand flexing of the walls of the screw holes in the bone plate.
According to one embodiment, the inner thread conically tapers at a cone angle of between about 5xc2x0 and about 20xc2x0. Preferably, the thread tapers at a cone angle of about 10xc2x0.
In the case where the inner thread is tapered, as discussed above, the thread may extend through a different angle when measured at the upper surface than when measured at the bone contacting surface. For example, when measured at the upper surface, the inner thread may extend through a first angle of between about 180xc2x0 and about 230xc2x0, while when measured at the bone contacting surface, the inner thread may extend through a second angle of between about 230xc2x0 and about 270xc2x0.
The first portion of the hole, and consequently the inner thread (if provided), may be oriented closer to the center or intermediate portion (as distinguished from the ends) of the plate than the second portion, thus avoiding any undesirable effects on the compression capability of the second portion. Thus, when the bone plate is used as a compression plate, the geometry of the second portion (compression portion) is not adversely affected by the presence of the internal thread.
According to another aspect of the present invention, at least one of the holes may be dimensioned and configured to receive a substantially spherical head of a bone screw and provide for compression of the fractured bone fragments. For example, according to one embodiment, the second portion of the elongated hole, discussed above, may include a concave, substantially spherical recess at the upper surface. The recess may be dimensioned and configured to accommodate the spherical head of a conventional bone screw. Such an arrangement may be especially useful when the bone screw is put in place eccentrically with respect to the hole, as is necessary for attaining compression of a fracture. Additionally, the second portion of the hole may flare outward in the area of the bone contacting surface to provide for increased angulation of the bone screw with respect to the bone plate.
According to another embodiment of the invention, the underside of the bone plate may be concave, thus allowing the plate to conform to the rounded cross-section of the tibia, femur, humerus, forearm bone, and other bones with which the present invention may be used. The concave configuration of the underside also allows a conventional bone screw to be inserted obliquely through the plate hole. This feature may be especially important when gripping a small bone fragment, which must be pulled against the plate.
The present invention is also directed to a bone plating system including at least one bone screw. The bone screw may have a screw-head that is configured and dimensioned to engage the circular portion of the above-described combination hole. For example, the screw-head may include a plurality of external threads disposed thereon that engage the internal threads of the bone plate.